1. Technical Field
This disclosure relates to a differential voltage sensing system and a method for achieving input impedance matching, and, in particular, to a differential voltage sensing system and method for sensing bio-potential signals.
2. Description of the Related Art
With the increased focus on personal health among people in recent years, and with the increasing life expectancy of the global population, applications for measuring and monitoring bio-potential signals such as the electrocardiogram (ECG) signal, the electroencephalogram (EEG) signal, the electromyogram (EMG) signal, and the electrooculogram (EOG) signal are frequently required, and the related products are increasingly popular. One widely-used bio-potential measurement apparatus measures signals from the human body via external skin patch electrodes adhesively attached to the subject's skin, and monitors physiological conditions of the heart or the brain via recording and further analysis.
The bio-potential signal acquired from the human body is quite small, and is easily subject to interference from the external environment or from other bio-potential signals from the test subject. Therefore, a differential voltage sensing system with a high common mode rejection ratio (CMRR) is suitable for measuring the small bio-potential signals so as to reduce the influence of noise on the measurement of the bio-potential signals. An input signal of the differential voltage sensing system usually has a common mode signal and differential mode signal. The main component of the common mode signal is noise, and the noise can be canceled by a differential amplifying method because the common mode noise signals are substantially equal in amplitude but invert of phase with one another. Therefore, the differential amplifying method can eliminate the common mode noises and amplify the small bio-potential signals.
However, the common mode noise signals cannot be eliminated when the input impedances of the differential voltage sensing system are not equal, and without elimination of the common mode noise signals the electrical signals are easily subject to distortion because of the interference noise. For example, skin morphology or adhesion of the electrode often results in differences of skin-electrode interface impedance so that the input impedances of the differential voltage sensing system are unequal. In particular, effective electrical signals cannot be captured from multi-pole EEG or multi-lead ECG signals when defective electrodes are present.
U.S. Pat. No. 6,208,888 discloses a voltage sensing system with input impedance balancing. The system comprises a feedback controller that adjusts an effective impedance associated with the second electrode according to a differential mode signal, a common mode signal, and an impedance associated with the first electrode. As a result, signals associated with each electrode undergo a similar degree of gain attenuation. However, since the input impedance balancing is achieved by varying the conversion characteristics of the circuit, an oscillatory noise is generated by such arrangement.
U.S. Pat. No. 5,233,985 discloses a cardiac pacemaker employing an operational amplifier output circuit for producing an electrical stimulating pulse. The circuit employs a variable resistor as a load for capturing signals and finds optimal common mode/differential mode matching points for performing signal measurement. However, such method employs manual operation to vary input impedances for matching, and controls signals by a virtual load.
Accordingly, there is a need to provide a differential voltage sensing system and a method for sensing bio-potential signals so that common mode noise signals can be eliminated using dynamic matching of the input impedances.